1. Field of the Invention The present invention relates to a supercharging pressure control device for a supercharger.
2. Description of the Related Art
In general, in an internal combustion engine for an aircraft which is equipped with a supercharger, the supercharging pressure is controlled by changing the degree of opening of a waste gate valve disposed in the exhaust gas passage of the engine.
An example of this type of supercharging pressure control device, is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-10686.
The device disclosed in the above publication controls a degree of opening of a waste gate valve of a supercharger (exhaust gas turbocharger) by using both of a feed forward control (open loop control) and a feedback control in order to adjust the supercharging pressure of a supercharged engine for an aircraft to a target supercharging pressure with a good response and good precision. Namely, the device of the above publication determines a waste gate valve opening setting (feed forward amount) for bringing the supercharging pressure substantially into coincidence with the target value from a predetermined numerical map based on the operating condition parameters of the supercharger other than the supercharging pressure such as an atmospheric pressure, engine speed (that is, an intake air amount), and engine air-fuel ratio. Further, in the device of the same publication, a waste gate valve opening correction amount (feedback amount) is calculated based on a deviation between the actually detected supercharging pressure and the supercharging pressure target value and the waste gate valve opening is set as a sum of the feed forward amount and the feedback amount.
In the supercharging pressure control device of the above publication, the opening of the waste gate valve is given as the sum of the feed forward amount determined by the open loop control without regard to the actual supercharging pressure and the feedback amount determined by the feedback control based on a difference between the actual supercharging pressure and the target supercharging pressure by using the values of the atmospheric pressure, engine speed, engine operating air-fuel ratio, etc. Therefore, when the actual supercharging pressure coincides with the target supercharging pressure at the waste gate valve opening setting (feed forward amount) given from the numerical map, the opening correction amount (feedback amount) becomes zero.
Since the feed forward amount is determined from the numerical map, and therefore, even in a case where the turbocharger intake pressure, intake amount, etc. change, a feed forward amount is set to a value corresponding the conditions after the change in a very short time.
The numerical map for calculating the feed forward amount is prepared based on a case where there is no (change in the engine performance and the turbocharger performance due to aging and, in addition, the engine is operated under standard operating conditions (for example, a standard atmospheric temperature, standard intake pressure loss of the turbocharger, etc.). Therefore, for example, when aging occurs in the performances of the engine and turbocharger, the feed forward amount obtained from the numerical map, i.e., the waste gate valve opening setting will deviate from the actual operating condition. Namely, if the waste gate valve opening is determined by only the feed forward amount, a steady deviation will sometimes be produced between the actual supercharging pressure and the target supercharging pressure.
On the other hand, the feedback amount is determined based on the deviation between the actual supercharging pressure and the target supercharging pressure. Therefore, even in a case where the performance of the turbocharger etc. changes, the feedback amount, i.e., the waste gate valve opening will be adjusted until the supercharging pressure coincides with the target supercharging pressure. However, when the rate of change of the feedback amount is made too large, the control becomes unstable, therefore it is necessary to set the rate of change of the feedback amount relatively small. For this reason, if the waste gate valve opening is controlled by only the feedback amount, there is a problem that a relatively long time will be required until the supercharging pressure is made to converge to the target supercharging pressure when the change in operating conditions of the turbocharger and the engine is large.
Therefore, in the device of the above publication, the above problem is solved by giving the sum of the feed forward amount and the feedback amount as the opening command value of the waste gate valve. Namely, in the device of the above publication, if the operating conditions such as the engine intake air amount and atmospheric pressure are changed, the waste gate valve opening is set according to the feed forward amount determined from the numerical map, and the supercharging pressure is immediately controlled to the vicinity of the target supercharging pressure. At the same time, the steady deviation of the actual supercharging pressure from the target supercharging pressure produced due to the change of the engine performance is corrected by a feedback amount set based on the difference between the actual supercharging pressure and the target supercharging pressure. Usually, the steady deviation is relatively small, and the feedback amount changes to the value appropriate for correcting the deviation in a short time. Therefore, by setting the waste gate valve opening as the sum of the feed forward amount and the feedback amount as described above, it becomes possible to make the supercharging pressure converge to the target pressure with a good precision while maintaining a good response with respect to change.
However, it is found that a problem occurs in which the response of the supercharging pressure control drops when the feed forward amount is determined from a numerical map set based on the standard operating condition of the engine as in the device of the above Japanese Unexamined Patent Publication No. 6-10686. Namely, as mentioned above, the feed forward amount is set based on the standard operating condition (standard atmospheric temperature and standard intake pressure loss), therefore where the conditions, for example, the atmospheric temperature and intake pressure loss, are largely changed from the standard condition, the feed forward amount per se also becomes a value largely deviated from the optimum value. Also in this case, the actual supercharging pressure can be correctly adjusted to the target supercharging pressure by the feedback amount, but if the deviation of the feed forward amount is large, the opening correction amount of the waste gate valve according to the feedback amount becomes large in response to this. For this reason, the converging time of the supercharging pressure becomes long, and there occurs a case where the control response is degraded. Particularly, in the case of an engine for aircraft, the change of the atmospheric conditions is large and the actual operating condition sometimes largely deviates from the standard condition and, therefore, the deviation of the feed forward amount also sometimes becomes large. Therefore, the problem explained above is liable to occur.